The pulse width modulator is a central element of any power conversion system. Most switching power converters are based on Pulse Width Modulation (PWM) as means to control efficient conversion between domains (DC or AC).
A typical power converter may include a PWM modulator, a switching power conversion stage, a filter and a control system. A prior art system of this type is described in U.S. Pat. No. 4,724,396 and by Mr. Attwood in Journal of the AES, November 1983. p. 842-853. However, PWM has a range of shortcomings also well known to the art, mainly due to the implementation of the carrier generation. This limits the system bandwidth and complicates design. Also, a stabile and robust control system design is difficult to implement.
In order to overcome these drawbacks, a non-hysteresis self-oscillating modulator (COM) was introduced in the applicant's international patent application WO 98/19391, and is illustrated in FIG. 1. Such a self-oscillating modulator eliminates the need of a carrier generator, with a range of advantages, described in detail in said document. The COM is implemented by two high frequency poles in the control loop 1 feeding back the signal from the power stage 2 to at least one summation point 3 in the converter. This local feedback loop 1 will therefore have a high gain and a high bandwidth.
A problem with this design is that the output filter of the switching power converter becomes more sensitive to load changes thus creating a dynamically unstable system. Due to the high sensitivity of the output filter loading, the system becomes unstable when a large load-step occurs, e.g. when the load is removed. This has required the use of e.g. a Zobel network in order to obtain damping of the output filter in low impedance loading situations.
In the case of multi loop systems, a global loop is defined as a loop 4 feeding back the output of the output filter 5 in the switching power converter. Such a global loop is limited when it comes to open loop gain and bandwidth, in order to avoid double oscillation. The limited feedback gain causes the output filter 5 of the switching power converter to have less compensation for filter non-linearities, thereby compromising the whole system performance.
Further, the structure of a multi-loop system with a modulator implemented in the local loop can be complex due to high order systems which will comprise many poles and zeroes in order to get the right open global loop characteristics.